Generally speaking, a frequency discriminator is a device for transforming frequency modulation into a low frequency demodulated signal and is usable in several applications, of which the most common include Intermediate Frequency (IF) demodulation and Automatic Frequency Control (AFC) of an oscillator, in which the discriminator is also caused to operate at the intermediate frequency.
At such "intermediate" frequencies which are generally several tens of megahertz (MHz) in a microwave transmitter or receiver, for example, it is conventional to use a "Travis discriminator" comprising two oscillating circuits tuned to two different frequencies F.sub.1 and F.sub.2 situated on either side of the IF, together with two diode detector circuits followed by respective filter circuits each including a capacitor connected in parallel with a load resistor. By connecting the two load resistors in series opposition (push-pull), and by taking the output voltage from across the terminals of the total load thus obtained, a voltage is obtained having the conventional amplitude-frequency characteristics A(f) shown in accompanying FIG. 1. A signal S.sub.f carried by a carrier wave F.sub.i and frequency modulated is thus transformed into an amplitude modulated signal F.sub.a by means of a discriminator having a response curve A(f) as shown diagrammatically in FIG. 1.
Microwave frequency discriminators have been implemented for a very long time, as can be seen from the book "Technique of Microwave Measurements" by C. G. Montgomery and published by McGraw-Hill Book Co., Inc., 1947, at pages 63 to 66. In those days the microwave circuits comprised magic-Ts, a metal microwave cavity, and waveguides having the property of reproducing an amplitude/frequency curve which is similar, at microwaves, to that of the Travis discriminator.
Such discriminators did not give rise to industrial implementations because they are difficult to make, bulky, expensive, and highly sensitive to temperature differences. These drawbacks had the effect of dissuading the person skilled in the art from using such circuits as demodulators or as AFCs, for example, and generally speaking as microwave discriminators. Indeed, over the last thirty years or so, these drawbacks have established a marked prejudice against such circuits so that frequency discriminators have been implemented in the path solely at IF.
Dielectric resonators for use with microwaves have been known since before 1940 (see "General of Applied Physics", Volume 10, June 1939, pages 391 to 398). Such dielectric resonators have been used since the beginning of the 60s in microwave applications: see in particular:
Proceedings IRE, Vol. 50, October 1962, pages 2081 to 2092;
IEEE transactions on Microwave Theory and Techniques, Vol. MTT-12, September 1964, pages 549 and 550; and
IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-13, March 1965, page 256.
More recently still, microwave oscillators have appeared in which the oscillator circuit is constituted by a dielectric resonator.
However, heretofore such facts have not been able to overcome the very marked prejudice of the person skilled in the art against using a frequency discriminator directly at microwave frequencies, and it has thus not occurred to the person skilled in the art that the very small size of a dielectric resonator could be used to implement a microwave discriminator which avoids the drawbacks of prior art microwave discriminators. It must also be added that dielectric resonators suffer from considerable temperature drift, as witnessed by the above-mentioned literature, and that the person skilled in the art is used to a frequency discriminator requiring an amplitude/frequency response curve which is particularly stable as a function of temperature.
Preferred embodiments of the present invention provide a microwave frequency discriminator which avoids the abovementioned drawbacks of prior art devices of this type.